Copal and Amber Varnish

An ingenious process for rendering hard copals soluble in oil without roasting them in the ordinary way has been recently devised and patented by H. Terrisse. It is based upon the fact that the solid hydrocarbon, naphthalene, a by-product in the manufacture of coal-gas, shares with some other organic bodies, liquids as well as solids, the property of dissolving hard resins when heated with them under pressure at a temperature not exceeding 300º C. In this way the greater part of the loss incurred when copals are roasted in the usual manner is avoided, while the discoloration of the torrefied resin does not occur. A mixture of 1 part of copal with 3 parts of naphthalene is taken, and then heated for an hour or two until complete solution has been effected. The mass is then transferred to a suitable still, when the naphthalene, being volatile, along with a little moisture and some oil of copal separated in the operation, are distilled off. Subsequent operations are two - the incorporation, under pressure, with the altered copal of the necessary proportion of linseed oil; and, secondly, the addition of spirit of turpentine to thin the product, which is pale in colour and yields a sound varnish.

Hard copals treated by this process suffer changes less profound than those brought about by the higher temperature involved in roasting them; they consequently retain more of their original qualities of hardness and toughness when finally turned into varnishes.

For the general use of painters in oil nothing more is wanted than true copal or amber oil-varnish, a drying oil, and a diluent. Of these three liquids a mixed medium in general use is compounded by taking equal measures of the three - varnish, oil, spirit of turpentine - and mixing them together in small quantities as required. But considering the large quantity of oil already associated with oil-pigments and present in copal or amber oil-varnish, one-third of oil in the medium seems a somewhat high proportion. I have proved by numberless experiments that it may be reduced with perfect safety to the permanence of the picture, although the manipulation and technique of a painter may demand the peculiar quality in a medium which oil in considerable proportion can alone supply. A formula which answers well is this:

2 measures of copal oil-varnish made from Sierra Leone or Zanzibar copal;

1 measure of poppy oil;

2 measures of oil of turpentine or oil of spike.

By substituting linseed oil for the poppy oil a more quickly-drying medium is obtained; still more rapid drying is secured by means of 'manganese' oil. With the same object in view, benzene may be used instead of oil of turpentine. This latter ought, of course, in all cases, to be one of the least resinifiable varieties obtainable.

If an amber or copal varnish containing no oil be available, a good medium may be prepared by mixing 3 measures of it with 2 measures of poppy or linseed oil, and 1 measure of oil of turpentine or of spike.